The present invention relates to a broadband voltage amplifier and more particularly to an amplifier adapted to process signals from an inductive component such as the magnetic reading head of a video tape recorder (VTR).
FIG. 1 very schematically shows such an inductive component 1 connected to the input terminal 2 of an amplifier A which amplifies the voltage between this input terminal 2 and an output terminal 3. This amplifier is liable to be a preamplifier constituting the input stage of an amplification circuit comprising other components.
FIG. 2 shows a conventional implementation of an amplifier A. It comprises:
a supply source 11, the first terminal 12 of which is connected to the output terminal through a resistor R1 and the second terminal 13 is grounded,
an NPN bipolar transistor T, the base of which is connected to the input terminal 2, the collector is connected to the output terminal 3 and the emitter is grounded, and
a resistor R2 connected between the collector and the base of transistor T.
The supply source 11 serves to polarize transistor T through resistors R1 and R2.
The voltage gain G1 of amplifier A is written: EQU G1=(.DELTA.V/VT)(R1/{[R1+(R2/.beta.)]}, (1)
where:
VT is a coefficient depending upon the temperature, PA1 .DELTA.V is the voltage difference between the voltage on the output terminal and the voltage on the input terminal, and PA1 .beta. is the gain of transistor T, equal to the ratio between the collector current IC and the base current IB.
With conventional constant resistor R1 and R2, it is clear that gain G1 depends upon .beta.. This gain .beta. is liable to substantially vary from one transistor to another. Indeed, it particularly depends upon the base thickness which is very low and that cannot be determined and reproduced with a high accuracy.
Then, in the circuit of FIG. 2, a resistor R2, associated during manufacturing to the gain .beta. of the transistor, has been provided. To manufacture this type of resistor, called pinched-base resistor, a first diffusion is carried out (base area) simultaneously with the base regions of the transistors. Then, a shallower additional area, which is of the same nature, is carried out simultaneously with the emitter regions of the transistors. The resistor value depends upon the thickness between the lower limits of the two diffusions (a pinch has been achieved), in the same way .beta. depends upon the thickness between the lower limit of the base region and the lower limit of the emitter region. Consequently, resistor R2 varies proportionally to .beta.. Then, gain G1 of the amplifier no longer depends upon .beta..
However, the formation of a constant voltage gain implies that the amplifier input impedance Z1 (Z1=R2.multidot.VT/.DELTA.V), which is proportional to R2, is no longer determined in a reproducible way from one amplifier to another.
This non-reproducibility of the input impedance is a drawback in case of application to the amplification of signals from the reading head of a video tape recorder. Indeed, the frequency of the signals applied to the input terminal of the amplifier is then liable to vary within a broad frequency band, for example between 100 kHz and 10 MHz. Then, the head impedance is substantially variable and of the same order of magnitude as the input impedance of the preamplifier. As a result, the effective gain of the preamplifier is not determined and is not reproducible from one manufacturing batch to another.
An object of the invention is to provide for a constant voltage gain within a broadband from one amplifier to another and with a determined and low input impedance at high frequency.